The spiny leaves make this weed easy to spot. Fortunately, spiny sowthistle is easy to control.

You can dig the long taproot with a dandelion fork (click for sources)  or uproot it with a Water Weeder.

A spot spray of organic non-selective weed killer or synthetic non-selective weed killer (click for sources) would work.

For your lawn, one of the broad spectrum herbicides like Bonide Weed Beater (click for sources), Ortho MAX (click for sources) or Bayer Season Long Control (click for sources) could give excellent control.

see Sowthistle pictures

spiny sowthistle

sowthistle (lt) vs dandelion (rt)